Difficulties with sleep have been inadequately studied in children with autism. In fact, disturbed sleep is one of the most frequent management problems reported by parents. Parents of children with autism describe more sleep problems in their children than parents of typically developing children or children diagnosed with other disabilities. Moreover, children who sleep poorly at night have been reported as being more likely to exhibit daytime behavior problems. Daytime sleepiness may interfere with the effectiveness of the child's educational and behavioral programs. Sleep problems also are significant stressors for families. Yet, no studies to date have objectively and systematically classified sleep disorders and assessed daytime sleepiness in children with autism compared to groups of children without autism. Over the past decade, autism has attracted increasing research and clinical attention related, in part, to the dramatic rise in the reported prevalence of this disorder. Nationwide, in the one-year period, 1997-98 to 1998-99, autism increased by 26% among children ages 6 to 21 attending school in the United States compared to an increase in all disabilities of 2.6%, specific learning disabilities of 2.3%, speech and language impairment of 1.0% and severe emotional disturbances of 1.9% (IDEA Report to Congress, 2000.) The specific aims of this study are to assess the prevalence, diagnostic characteristics and daytime concomitants of sleep disorders in children with autism compared with a matched group of developmentally delayed children without autism, and a group of age-matched developmentally typical children. The associations between neurodevelopmental diagnosis, developmental / chronological age, sleep disorder classification and degree of daytime impairment including daytime sleepiness, derived from multiple measures, will be examined. We hypothesize that 1) DSM-IV sleep disorders in children with neurodevelopmental disorders can be diagnosed reliably using a combination of methods; 2) Sleep disorder diagnoses will be stable over a 6-month period; 3) Children with AUT will have more sleep-onset (phase delay) disorders than children with DD, and will have more severe disorders than typically developing children; 4) DD children without autism will have more fragmented sleep (insomnia) than children with AUT and fewer phase delay disorders, and will have more severe disorders than typically developing children. 5) Children with severe sleep disorders in all groups will demonstrate more daytime dysfunction (inattentiveness and hyperactivity) than children without a sleep disorder, substantiating the need for developing specific treatments for sleep disorders, and 6) Sleep disorder severity in all groups will be related to levels of parenting stress.